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GRaND Seeks Subsurface Water Ice on Ceres

March 22, 2016
Tucson, Ariz. -- The Gamma Ray and Neutron Detector (GRaND) aboard NASA’s Dawn spacecraft is mapping the elemental composition of Ceres in a low altitude orbit, about 240 miles (385 kilometers) above the surface of the dwarf planet.
“GRaND is in excellent health and operations have gone smoothly during the Ceres encounter, enabling us to acquire a high quality data set,” said PSI Senior Scientist Thomas Prettyman, Dawn mission co-investigator and lead for GRaND. 
Neutrons and gamma rays produced by cosmic ray interactions with surface materials provide a fingerprint of Ceres’ chemical makeup. The data will be analyzed to determine the concentration of chemical elements within the topmost 3 feet (1 meter) of Ceres’ surface.
Data relevant to the possibility of subsurface ice are emerging from GRaND, which began acquisition of its primary data set in December. In Dawn's lowest-altitude orbit, the instrument has detected fewer neutrons near the poles of Ceres than at the equator, which indicates increased hydrogen concentration at high latitudes. As hydrogen is a principal constituent of water, water ice could be present close to the surface in polar regions.
"Our analyses will test a longstanding prediction that water ice can survive just beneath Ceres' cold, high latitude surface for billions of years," Prettyman said.
High-energy cosmic rays produce neutrons and gamma rays when they interact with materials in the outermost layer of the cerean surface. In addition, gamma rays are made by the decay of radioelements, such as potassium and thorium, found in rocks and soil.  A portion of the radiation escapes into space.  In low altitude orbit, GRaND can detect radiation originating from Ceres. The spectrum of gamma rays and neutrons measured by GRaND provides information about surface elemental composition.  The chemical data contain clues about Ceres’ origins and evolution.
Visit for a map of neutron counting data acquired by Dawn’s GRaND in the lowest altitude orbit.
Prettyman's work is funded by a grant from the Jet Propulsion Laboratory, managed by the California Institute of Technology, under contract with NASA. GRaND is managed by the Planetary Science Institute.
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